In several animal species, it has been shown that the visual environment can affect the development of the eye. For instance, tree shrews raised from near birth until adulthood with one eyelid closed become myopic in the closed eye. This myopia is not due solely to the lid closure because, if the animals are raised in total darkness, the closed eye does not become myopic, relative to the open eye. It also is not due to genetic factors because, in a single animal, the eye that is subjected to the environmental treatment becomes myopic while the control eye does not. We propose experiments that are aimed at determining whether accommodation or extraocular muscle activity is the cause of this effect. We also propose to examine whether patterned light information is used to regulate the growth of the eye in this animal. Experiment 1 will examine the growth of the experimentally myopic eye as it develops differently from its normal fellow eye to gain clues about how environmental factors alter the growth of the eye to produce the myopia. In Experiment 2 we will test whether accommodation and/or activity of the extraocular muscles are causal factors in lid closure-induced myopia. These two experiments will help us to better understand the mechanisms by which the visual environment affects ocular growth. In Experiment 3 we will determine whether recovery occurs from myopia produced by short-term lid-closure and, if so, whether this is due to selective decreased growth of any particular ocular component. In Experiment 4 we will riase tree shrew pups with minus-power soft contact lenses. If there is active regulation of ocular growth to achieve emmetropia, the animals should vary the growth of the eye in correspondence with the power of the lenses. These experiments will significantly advance our understanding of the mechanisms by which the visual environment affects the development of the eye. If we can learn how the environment can produce ametropia in the eyes of tree shrews, a species which is closely related to primates, it will then be reasonable to ask whether these same mechanisms may affect ocular development in human beings.